The singular character of the surface of carbon derived from its microporous structure, are well known, among them, character of molecular sieve is worth noticing which the selective adsorptive capacity changes depending on the size of the micropore of the surface. The pressure swing adsorption process is one of the most important uses of a carbon molecular sieve which is adapted to separation of gaseous mixture by making use of the difference of the character of selective adsorption.
In Japanese Patent Publication No. 38-25969, wherein, a Principle and Process of pressure swing adsorption and one of its application example--separation of nitrogen and oxygen gas from air with activated carbon--are disclosed. The surface of carbonaceous material naturally has more or less the property of a molecular sieve, derived from its microporous structure. In 1948, P. H. Emmett first prepared a carbon molecular sieve by dry distillation of polyvinylidenchloride resin. (P. H. Emmett: Chem. Rev. vol. 43, P. 69).
Usually, the diameter of micropores on the surface of carbon prepared by conventional processes is usually greater than 10-20 .ANG., so micropores of this size are too large to separate molecules of small size, for example separation of nitrogen and oxygen gas from air, namely the property of molecular sieve is inadequate. Several processes of manufacturing carbon molecular sieve which is applicable to separation of oxygen and nitrogen gas from air, have been disclosed, wherein micropores are partially packed with fine carbon particles and their size become narrow, thus the property of a molecular sieve is increased. For example, such a process is disclosed in Japanese Patent Publication No. 49-37036, wherein small amounts of prepolymer of phenol resin or furan resin are added to activated carbon, polymerized, carbonized by heating at 400.degree.-1,000.degree. C., thus micropore on activated carbon is partially packed with carbonaceous material and given the property of molecular sieving. In Japanese Patent Publication No. 52-18675, coke of which volatile material content is less than 5%, is heated to 600.degree.-900.degree. C. in a furnace, then hydrocarbon vapour, such as benzene, toluene and producer gas, is introduced therein, soot generated by decomposition of hydrocarbon partially packs the micropores of coke, thus a molecular sieve made of coke is formed. In this process, decomposition of hydrocarbon vapour in the furnace is considered to be an indispensable requirement. In Japanese Patent Application Laid Open No. 49-106982, a method to narrow the size of micropores of coke is described, wherein an organic compound in vapour or in solution is adsorbed onto the surface of coke, thus, the micropore is partially packed. In Japanese Patent Application Laid Open No. 56-130226, a method to narrow the size of the micropores of carbon of size of more than 0.5 nm, is described wherein a concentration of hydrocarbon in vapour of less than 2%, is adsorbed on the surface of carbon and heated, thus the soot generated by the decomposition of hydrocarbon adhered to its surface and narrowed the micropores. In this treatment, hydrocarbon is necessary to have molecular size of more than 0.5 nm such as methane, ethane and styrene. This method comprises to decompose gaseous hydrocarbon and precipitate its soot in the micropores of activated carbon or to adsorb gaseous hydrocarbon in these micropores then decompose in it, consequently narrows their size with soot, thus improves the carbon molecular sieving property. In this process, it is necessary to introduce the gaseous hydrocarbon into the furnace and decompose, precipitate soot on adsorbent, so with this process it is complicated and difficult to obtain a high quality carbon molecular sieve stably. At the same time, cost of the product becomes expensive.
In Nenryokyokaishi vol. 60, No. 654, page 859-864 (1982), a method to improve the molecular sieving property of Yallourn char is described, wherein, 4-5% of exhaust liquor of sulfite pulp and coal tar pitch are added to Yallourn char then pelletized, heated to 600.degree.-700.degree. C. at the speed of 10.degree. C. min.sup.-1 in nitrogen gas and maintained for 1 hour. In this process, the hardness of the pellet is inadequate because of the exhaust liquor of sulfite pulp used as a binder, and it is difficult to maintain the molecular sieving property of the product constantly.